1. Field of the Invention
The present invention relates to a device for correcting an unbalance of a propeller shaft for transmitting drive force to the rear wheel of a motorcycle; specifically, an unbalance correction device of a propeller shaft capable of securing a considerable amount of correction of unbalance of the shaft, reducing the generation of wind roar, air resistance, noise and the like while the motorcycle is running, and preventing the adverse effect of welding heat caused by welding and fixing the balance weight from exerting on a vibration absorbing rubber member.
2. Description of the Related Art
As the drive transmission system from the engine to the real wheel of a motorcycle (or a motorbike), generally speaking, there is a chain-based transmission system and a shaft-based transmission system. With the shaft-based transmission system, the rotation of the output axis of the engine is transmitted to the shaft, and the rotation of the shaft is transmitted to the rear wheel. This shaft is usually configured from an inner shaft and an outer shaft, and provided with components such as a universal coupling (joint). Moreover, a vibration absorbing elastic member is mounted between the outer shaft and the inner shaft.
As conventional technology, this kind of shaft is described in Japanese Utility Model Laid-open Publication No. H5-94555 and Japanese Utility Model Laid-open Publication No. S55-61637. The reference numerals are indicated in parentheses upon explaining Japanese Utility Model Laid-open Publication No. H5-94555 and Japanese Utility Model Laid-open Publication No. S55-61637 below. Japanese Utility Model Laid-open Publication No. H5-94555 relates to a truck and not a motorcycle, but discloses a propeller shaft for transmitting the rotary torque from the transmission to the rear axle. Incidentally, the reference numerals are indicated in parentheses upon explaining Japanese Utility Model Laid-open Publication No. H5-94555 below. Japanese Utility Model Laid-open Publication No. H5-94555 discloses a method of correcting the unbalance by welding a first balance piece (35) and a second balance piece (36) to the outer periphery surface of a pipe (24) and a yoke sleeve (23).
In Japanese Utility Model Laid-open Publication No. H5-94555, the first balance piece (35) and the second balance piece (36) are rectangular shaped steel plates, the yoke sleeve (23) is a cast structure, and the pipe (24) is made of iron. The first balance piece (35) and the second balance piece (36) are formed from a curved plate (37) and a protruding part (38) as shown in FIG. 3, and the protruding part (38) is welded thereto. This kind of propeller shaft, as described in Japanese Utility Model Laid-open Publication No. H5-94555, has the three main functions of “transmission of rotational torque,” “absorption of position variation” and “blocking of rotational vibration.”
In Japanese Utility Model Laid-open Publication No. H5-94555, the propeller shaft is configured from numerous components. Since there is variation in the precision of the individual components and variation in the subsequent assembly process, if the components are assembled as is, in most cases the weight distribution becomes uneven; that is, an unbalance occurs. Thus, in order for the propeller shaft rotating a high speed to sufficiently satisfy the foregoing three functions (“transmission of rotational torque,” “absorption of position variation” and “blocking of rotational vibration”), it is important to correct the unbalance of the propeller shaft. Meanwhile, Japanese Utility Model Laid-open Publication No. H5-94555 welds the first balance piece (35) and the second balance piece (36), and much of the welding heat is transmitted from the balance pieces to the propeller shaft.
The propeller shaft is internally provided with a damper or the like made of rubber or other materials in order to achieve the blocking of rotational vibration among the foregoing three functions. Thus, there is a possibility that the function of the damper or the like may deteriorate due to the thermal stress of components of the propeller shaft or the hardening or rubber or other materials caused by the welded parts. In order to correct the unbalance of the propeller shaft, if the balance piece is welded and fixed, the components of the propeller shaft will be subject to thermal stress due to the welding heat, and new unbalance may arise in the propeller shaft. In order to overcome these various drawbacks, the location of welding the balance piece is limited to an extremely limited range at the end in the axial direction which is farthest from the damper or the like, and it is also necessary to reduce the welding amount as much as possible in order to reduce the welding heat.
Thus, in Japanese Utility Model Laid-open Publication No. H5-94555 also, only the protruding part (38) is welded as a small area at the end of the yoke sleeve (23). Nevertheless, as described above, since the range that the balance piece can be welded is limited to the end part in the axial direction, as shown in FIG. 3 of Japanese Utility Model Laid-open Publication No. H5-94555, if the balance piece is formed in a simple square shape, the size of the balance piece cannot be enlarged to be a given size or more, and there is a problem in that the amount of correction of the unbalance cannot be secured sufficiently. Specifically, if the balance piece is enlarged too much, the welding point of the balance piece will extend to the range where welding should not be performed; that is, to an area where the damper is mounted, and the damper will deteriorate due to the welding heat.
Moreover, in Japanese Utility Model Laid-open Publication No. H5-94555, two protruding parts (38) are arranged and formed in a single line on the balance piece (35) along the longitudinal direction of the curved plate (37). In addition, a welding electrode is made to come in contact with the propeller shaft body (22) and the balance piece (35) by causing the longitudinal direction of the balance piece (35) to coincide with the axial direction of the propeller shaft body (22), and the protruding parts (38) are thereby welded to the propeller shaft body (22) (refer to FIG. 9A, FIG. 9B). Upon performing the welding, since the balance piece (35) is in contact with the protruding parts in a single line when viewed from the front in the axial direction of the propeller shaft body (22), the balance piece (35) is unable to obtain stability in the width direction.
Accordingly, it is difficult to accurately fix the balance piece (35) to the propeller shaft body (22) in the width direction, and in certain cases, it is sufficiently possible that the balance piece (35) will fall and be fixed. For instance, FIG. 9C shows the balance piece (35) being fixed to the propeller shaft body (22) in its width direction in a state of falling (being tilted) at an angle θ upon performing the projection welding. If the propeller shaft body (22) to which the balance piece (35) is fixed in the foregoing manner is rotated, the following problems will arise.
Foremost, let it be assumed that the propeller shaft body (22) rotates in a clockwise direction when viewed from the front in the axial direction. Meanwhile, the aerial flow of the circumferential direction of the axis will be generated in the opposite direction relative to the rotating direction of the propeller shaft body (22). If the curved plate (37) of the balance piece (35) falls to the right side relative to the perpendicular line passing through the diameter center when viewed from the front in the axial direction of the propeller shaft body (22), the aerial flow will pass above the curved plate (37) of the balance piece (35), and a load (+)F that yields pressed the surface of the left side of the curved plate (37) in the width direction toward the propeller shaft body (22) side will be applied (refer to FIG. 9D, FIG. 9E).
Moreover, contrarily, if the curved plate (37) of the balance piece (35) falls to the left side relative to the perpendicular line passing through the diameter center when viewed from the front in the axial direction of the propeller shaft body (22), the aerial flow will enter the back face on the right side of the curved plate (37) in the width direction and flow in a manner of colliding with the rear surface side of the curved plate (37), generate an air swirl, this becomes wind pressure resistance, and a load (−)F that attempts to peel the balance piece (35) from the propeller shaft body (22) is applied (refer to FIG. 9F, FIG. 9G).
Thus, with the balance piece (35), since the protruding parts (38) are formed in a single line along its longitudinal direction, and the column direction of the protruding parts (38) is along the axial direction of the propeller shaft body (22), regardless of whether the balance piece (35) falls toward the left or right when viewed from the front in the axial direction of the propeller shaft body (22), either the foregoing load (+)F or (−)F will be applied, and the balance piece (35) will be easily subject to rattling due to the vibration of the curved plate (37) since both ends of the curved plate (37) in the width direction are in a free state (refer to FIG. 9E, FIG. 9G).
Next, Japanese Utility Model Laid-open Publication No. S55-61637 discloses integrally forming, by bending, a protruding line (3c) protruding outward with an annular plate (3) in a propeller shaft, and fixing a balancing weight (4) to the protruding line (3c). Indeed, the amount of unbalance correction can be increased further by disposing the balancing weight (4) outward of the radial direction. However, generally speaking, the propeller shaft is disposed in the front and back directions in a substantially parallel manner relative to the traveling direction of the vehicle, and the propeller shaft rotates at high speed.
Thus, since the wind pressure will directly hit the balancing weight (4), there is a possibility that wind roars will occur or that the balancing weight (4) will become air resistance. In addition, if the balancing weight (4) is simply fixed to the tip of the protruding line (3c), since the tip side will be heavier, the balancing weight (4) will sway like a see-saw from the root of the protruding line (3c), and may become the source of generating noise. This is a phenomenon that occurs because the propeller shaft rotates at high speed and the wind pressure hits directly at a random size and direction. Moreover, since the rotating speed of the propeller shaft increases and decreases at random, the swaying of the balancing weight (4) will also be random.